The primary objective of the proposed study is to determine whether exposure to a 60-Hz magnetic field in the home sleeping environment produced by an electric blanket is associated with a decrease in nocturnal urinary melatonin levels and an increase in nocturnal urinary levels of luteinizing hormone (LH), follicle stimulating hormone (FSH), and estradiol in a sample of healthy women of reproductive years. Secondarily, this study is designed to investigate the relationship between nocturnal urinary melatonin levels and nocturnal urinary levels of LH, FSH, and estradiol, regardless of nighttime exposure to magnetic fields. At least 180 women will be recruited as volunteers from the Seattle metropolitan area. Eligible for participation will be any woman between the ages of 20 and 45 who is a resident of King County, who has not been previously diagnosed with breast cancer, who reports regular menstrual periods, who is not taking oral contraceptives or hormone preparations, and who normally sleeps with an electric blanket during the winter months. This investigation will employ a cross-over design which treats each participant as her own control. Data collection will take place during the winter months, when an electric blanket would typically be used and will involve a three month period for each subject. The first month is designed to evaluate menstrual cycle regularity. The second and third months are each comprised of a washout period followed by either an exposure period (electric blanket use beginning the day of ovulation through Day 7 post ovulation) or a non- exposure period (no electric blanket use beginning the day of ovulation through Day 7 post evaluation). Half the subjects will be assigned to exposure followed by non-exposure while the other half of the subjects will be assigned the reverse order, non-exposure followed by exposure. Using commercial kits, ovulation will be determined for each month. During Months 2 and 3, magnetic field levels will be characterized in each subject's sleeping environment from Day 3 post ovulation through Day 7 post ovulation, and nocturnal urine will be collected on Day 7 post ovulation. Urine samples will be assayed for the primary urinary metabolite of melatonin, 6-sulphatoxymelatonin, and levels of LH, FSH, and estradiol. Information on the subject's medical history, medication use, and characteristics of the electric blanket will also be collected.